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Flashcards in CVS S7 - The ECG Deck (38):

Describe the spread of excitation through the heart

You should be able to draw a diagram of this

Activity starts at the SAN and depolarisation spreads over the walls of the atria to the AVN

At the AVN there is a 120ms delay

Excitation then spreads down the septum through the bundle of His and the left and right bundle branches then out over the ventricular myocardium

Excitation through the myocardium progresses from the endocardial to epicardial surface, at this point, the entire ventricle is depolarised

There is a delay before ventricular repolarisation of 280ms

Repolarisation spreads in the opposite direction to depolarisation, from epicardial to endocardial surface


What would an extracellular electrode placed near a myocardial cell see during each contraction?

Why does this differ from what an intracellular electrode sees?

Sees only two signals for each action potential/systole:

- One for depolarisation

- One for repolarisation

Electrodes outside the cell only record CHANGES in membrane potential


How does what an electrode see depend on the direction of depolarisation and repolarisation?

If depolarisation is spreading:

- Toward electrode then upward signal

- Away from electrode then downward signal

If repolarisation is spreading:

- Toward electrode then downward signal

- Away from electrode then upward signal


What determines amplitude of signal seen by an extracellular electrode?

Amplitude is larger when:

- There is more muscle depolarisaing/repolarising

- The spread is at a smaller angle to the electrode view (largest when directly towards/away)


What are the different waves found on an ECG and what electrical events do they correspond to?

P wave - Atrial depolarisation

Q wave - Septal depolarisation spreading to ventricles

R wave - Main ventricular depolarisation

S wave - End ventricular depolarisation

T wave - Ventricular repolarisation


Describe how the P wave's form is produced

What happens following the P wave?

Atrial depolarisation produces a small upward deflection

Small and upward because it's moving toward the electrode, but not directly

Following the P wave:

- 120ms delay before QRS complex


Describe how the Q wave's form is produced

Depolarisation spreads down the septum then out across the axis of the heart, producing a small downward deflection

Small and downward because it's away from the electrode, but not directly


Describe how the R wave's form is produced

Depolarisation spreads thorough the ventricular muscle along an axis slightly left of the septum producing a large upward deflection

Large and upward as there's lots of muscle and depolarisation is moving directly toward the electrode


Describe how the S wave's form is produced

What happens directly after the S wave?

Depolarisation spreads up the ventricles towards the atria producing a small downward deflection

Small and downwards as it's moving away from the electrode, but not directly

Afterwards, there is a delay of 280ms before the T wave


Describe how the T wave's form is produced

Repolarisation spreads from epicardial to endocardial surfaces producing a medium upward deflection

Upward because repolarisation is moving away

Medium sized because repolarisation timing in different cells is dispersed


Label each of the black boxes

From left to right

P wave

Q wave

R wave

S wave

T wave

At the top:

QRS complex


Where is atrial repolarisation on an ECG trace?

It can't be seen as it gets lost in the QRS complex


Describe how the R wave would change if an electrode is moved around the heart

Viewing directly in line with the cardiac axis results in a large upward deflection

Viewed at a 90 degree angle to the cardiac axis there is no signal

Viewed at a 180 degree angle sees a large downward deflection

Varies predictably between these points


How many electrode comprise are involved in a normal ECG reading?

How many are recording?

Where are these electrodes? (In the broadest of terms)

10 electrodes in a 12 lead ECG

9 recording, right lower limb is neutral

4 Limbs

6 Chest




Where are each of the limb electrodes placed?


Red - Right upper limb

Yellow - Left upper limb

Green - Left lower limb

Blue - Right lower limb


What are all the limb leads called?

State the equivalent single electrode views of all the leads

All veiwing the heart in the coronal plane, all angles given relative to Lead I, which views the heart directly from the left


Lead I - 0 degrees

Lead II at +60 degrees

Lead III at +120 degrees

aVL at -30 degrees

aVR at -150 degrees

aVF at +90 degrees



What are the limits of the normal cardiac axis?

Between aVL and aVF ( -30 to +90 degrees)


Describe the placement of each of the chest electrodes

V1 - Right 4th intercostal space next to the sternum

V2 - Left 4th intercostal space next to the sternum

V3 - Directly between V2 and V4

V4 - 5th intercostal space along the left midclavicular line

V5 - Level with V4 at the left anterior axillary line

V6 Level with V5 along the left midaxillary line (directly under the midpoint of the armpit)


What leads correspond the the chest electrodes?

What views do they provide?

Leads correspond to each electrode (V1, 2, 3, 4, 5, 6)

They provide horizontal views of the heart


Describe how leads are amplified

One electrode is used as positive

One is negative (can be directly opposing positive or offset)

Negative signal is inverted and the signals are combined

The signal is then amplified further

If the negative as opposing positive, we get an amplified view directly from the positive (E.g. Lead I)

If the negative was offset we get and amplified view from between the positive and equivalent positive of the negative (directly opposite the negative) (E.g. Lead II and III)


Label each of the black boxes with the corresponding limb lead, include degrees offset from Lead I

Starting from the lead directly to the left and going clockwise

Lead I at 0 degrees

Lead II at +60 degrees

aVF at +90 degrees

Lead III at +120 degrees

aVR at -150 degrees

aVL at -30 degrees


How do you calculate a regular heart rate from an ECG?

Where do you read this rate from?

All ECG machines run at a standard rate of 300 small squares per minute

To find the rate, divide 300 by the number of squares in the R -R interval.

The rate is commonly read from the 'rhthym strip' but can be read from any lead



How would you calculate an irregular rhythym from an ECG?

A larger interval is used than a single R- R interval

For example, multiply the number of beats in 10 seconds by 6


What are ventricular ectopic beats and how do they present in an ECG?

Ventricular cells that gain pacemaker capability can cause ventricular contraction before underlying rhythym depolarises the ventricles, the ectopic beat

The resultant ECG often appears wider and taller than the underlying rhthym

May occur every other beat, every third beat, every fourth beat etc. or in couplets or triplets.


How does atrial fibrillation appear on an ECG?

P waves replaces by fibrillatory waves

Variable ventricular rate


What is ventricular fibrillation?

How does ventricular fibrillation appear on an ECG?

Uncoordinated contraction of the ventricular myocardium

Rapid irregular waves of varying amplitude


Describe frist degree heart block

P-R interval is legthened from its normal 120 - 200ms

There is a conduction delay through the AVN but all electrical signals reach the ventricles


Describe type 1 second degree heart block

The P-R interval is erratic 

P-R interval follows a pattern of elongating until a QRS complex is dropped then resetting to normal and progressively elongating again

Some but not all atrial beats reach the ventricles


Describe type 2 second degree heart block

Electricla conduction sometimes fails to pass through the AVN or bundle of His

P-R interval is constant but not all atrial beats get through to the ventricles


Describe third degree heart block

No association between the atrial pace and the ventricular pace

Normal atrial excitation doesn't reach the ventricles which are then controlled by ectopic pacemaker cells resulting in two separate rhythms


Describe how a bundle branch block might change the shape of an ECG trace

Damage to the conduction pathways alters the route of spread of depolarisation

Lengthens and changes the shape of the QRS complex

Many variations depending on location


What is the electrical axis of the heart?

Relates to the main spread of depolarisation through the wall of the ventricle (R wave)

Combination of depolarising right and left ventricles generates a single vector normally pointing slightly to the left (as there is more depolarising muscle in the left ventricle)



What will cause changes in the electrical axis of the heart and why?

Will change in response to changes in the relative amount of depolarising muscle in each ventricle

Increase in muscle in the left of the heart will create a left axis shift

Increase in muscle in the right of the heart will create a right axis shift


How is electrical axis of the heart determined?

Describe both normal and abnormal axis determination


Look at the limb leads

Normal axis = positive Lead I and III deflection

Left axis deviation = Positive Lead I deflection, negative lead III deflection

Right axis deflection = Negative lead I deflection, positive lead III deflection


How does damage or stress to the myocardium alter electrical conductivity?

Answer in broad terms

Stressed, dying or dead myocardium generates current flows during systole

These produce extra signals in the S-T segment


What is the effect on the heart of temporary ischaemia?

Reduced blood flow leads to angina and ST depression in most cases


Explain the most common changes in the ECG response during a myocardial infarction?

Dying tissue generates injury currents often leading to ST elevation


Pathological Q waves (Wider than 1 small square and more than 2mm deep) present in full thickness MI and remain after resolution

Inverted T waves


How might you determine the location of a myocardial infrction from an ECG?

The view with the most prominent abnormality helps you identify:

- Which part of the heart is affected and how badly

- Which coronary artery is blocked 

- Whether the whole thickness of the ventricular wall is affected